When a canister is emptied, sooner or later as much air has to flow in as liquid flows out from its pouring nozzle. The most common canisters are 5 liter canisters, 10 liter canisters and 20 liter canisters. Of course, these liter figures are only approximate and need not be precisely correct. The 5 liter and 10 liter canisters of plastic are only available with screw closure, because a claw closure would be too elaborate in this case, and for other design reasons. In the case of the 5 liter canister, often there is the chance of glugging, which disrupts the pouring operation and is due to the fact that when there is sufficient negative pressure in the canister the air is sucked-in through the pouring spout, during which time no liquid, or very little liquid, can flow out from the pouring nozzle.
There are a considerable number of pouring nozzles for 5 liter and 10 liter canisters which have air return tubes inside. If no liquid is to pass through these, but air is to flow in, they must be at the top during pouring. There are two approaches as to how to accomplish this:
(a) The manufacturer of the canister always produces the external screw thread--and has done so for many years--such that the start of the thread is always in the same place. With such a canister he supplies a pouring spout, the screw socket with internal thread of which, firmly connected to the spout, has such an internal thread that, with pouring spout tightened liquid-tight, the air tube is at the top. This has the disadvantage that production has to be kept very accurate. In addition, the pouring spout--even if it is injection-molded--is nevertheless so compliant in the thread region that the screwing movement cannot be stoped exactly when the air tube is at the top. The angled-off end region of the pouring spout then faces to the side. Such inaccuracies in the screwing end stop may also be due to the fact that the seal wears out, is lost or the like.
(b) Two-part pouring spouts are produced in which the screw ring is a separate part. Then the spout can in fact be held firm in the correct position. The start of the external thread is thus not critical in this case. There is the disadvantage, however, that many people are uninformed about flow conditions and, in spite of the actually existing possibility of doing it correctly, the air tube is not at the top. In addition, a two-part design, with all its disadvantages such as sealing points, stock keeping, loseability, etc., suffers considerable disadvantages. The two parts also have to be produced by different technologies. The screw socket must be injection-molded, while the actual spout is usually blow-molded. Examples of the different pouring spouts which can be screwed on to screw-on closures of plastic canisters are found, for example, in West German Utility Model 1808610, in German Patent Specification 3508320, in German Patent Specification 1930906.
In the case of 20 liter canisters, the air return can only be disregarded as far as low-grade canisters are concerned. For industrial purposes, military purposes or the purposes of emergency services, the canisters must have an air tube which reaches from the pouring nozzle into the rear regions of the canister. The canister's own venting opens out in the end face of the pouring nozzle, and if the pouring device likewise has claws, the spatial relative arrangement of a certain place on the end face of the pouring nozzle and any air tube there may be in the pouring spout is provided as a matter of course. The air flowing in then reliably flows from the mouth of the spout to the end of the canister's own air tube.
However, claw closures are expensive. Some people do not understand their operation. They have more individual parts than screw closures. A screw-on spout has, however, the threading disadvantages mentioned above in relation to 5 liter and 10 liter canisters, and consequently it is not possible to empty a screw-closure canister in a simple way. This is of much greater disadvantage with 20 liter canisters than with 5 liter and 10 liter canisters, because in the case of the former it takes much longer until it is empty.